The ones on the left are the later so-called 'low-inertia' steel contact set, set up for a magneto that turns anti-clockwise from the driven end - so the contact breaker turns clockwise looking from the clutch side of the bike. The ones on the right are a brass backplate set, as used until the steel assembly came along. The brass sets are handed - that set is for a mag that turns clockwise from the driven end. Which would suit eg Vincent, some Enfield twins, and some Douglas, as well as a large number of 4 cylinder Lucas mags on cars.
The adjustment of the steel set is car-like, and involves slackening the centre screw (which is slightly different from the one used with the brass part) and the other little screw that holds the steel bit to the backplate; the brass type involve just the lock-nut and screwed post on the fixed point. A steel assembly will go on any ringcam Lucas mag, but with brass, it has to be the right rotation.
One other difference between the 2 types is that with the steel assembly the 'live' point is the moving one, while on the brass set the live point is the fixed one and the sprung one is earthed. This matters if the spring blade should brush against the ringcam on the way round - the steel set will be shorted out (so no sparks), the brass sort won't - but might wear its outer spring blade away until it breaks!
Cheers, Bill
